This invention relates generally to the use of TDR (Time Domain Reflectometry) testing for circuit faults, analysis, extracting and modeling electrical characteristics of interconnects on PCB (Printed Circuit Boards), IC Packages (chip-to-chip, chip-to-board, and board-to-board), subsystems and other semiconductor interconnect circuitry, and more specifically, to a probe having a built-in reference ground plane for quick, consistent TDR measurements for such testing.
TDR has traditionally been used to evaluate and test all types of transmission lines, including power lines, telephone, cable TV, optical fibers, etc. However, since substantially any type of electrical connection for transmitting signals is a transmission path or transmission line, the concept is now being used for analyzing and testing connections and signal paths on all types of electronic circuitry including semiconductor circuitry and micro circuitry. As the electrical behavior of a semiconductor package, multi-chip module, and circuit board interconnect becomes increasingly more important in determining the behavior of digital and analog systems due to the increasing speed and power requirements and decreasing operating voltages of these circuits, TDR testing has become more common. As an example only, one impediment to the development of the next generation of micro processor modules will likely be the lack of power supply stability caused by the interconnects between the micro processor chip, its power supply by-pass capacitor and its power supply voltage regulator. The reason that interconnects are becoming more important for the power supply stability of future micro processors is that power requirements have changed. For example, the power supply voltage is changing from 3.3 volts to 1.1 volts for future systems. Further, the voltage compliance or tolerance requirements are changing from 5% to 2%. Interconnects are also becoming more important for power electronic systems because new power device technologies have increased both the speed and current capacities.
As performance requirements for computer and communication systems grow, the demand for high-speed printed circuit boards (PCBs) and interconnects increases. It is not unusual to find that speeds on the order of 1G bit per second need to be supported by standard printed circuit board technologies. The rise time of these signals can be in excess of 100 PS (Pico Seconds). At these speeds, the relatively short interconnections on printed circuit boards behave as distributed elements or transmission lines, and reflections due to impedance mismatching is a typical signal integrity problem. Vias between layers and connectors on a board create discontinuities that further distort these signals. To accurately predict the propagation of signals on a board, one needs to determine the impedance of the traces on different layers and then extract models for board discontinuities. Time domain reflectometry measurements are the measurement approach of choice for this type of characterization worked. Plus, based on TDR measurements, a circuit board designer can determine characteristic impedance of board traces, compute accurate models for board components and predict board performance more accurately.
A major focus of time domain reflectometry is characterization of interconnect-transmission line properties. With the absence of adequate metrology standards for characterizing interconnects, the electromagnetic wave propagation properties of interconnects are often approximated using analytical or computational methods which do not adequately predict the non-ideal behavior of real interconnects. In these methods, there are some fundamental limitations because the physical properties, parameters and structures of the conductor material are not generally known and an assumption must be made in computations. For example, the transmission line properties of structures such as leads or wire bond interconnects inside device modules or components are almost impossible to calculate accurately because they are physically inaccessible after being packaged. Therefore, a measurement based method such as TDR is ideal for characterizing the transmission line properties of the interconnect parameters. Time domain measurements are also useful because the interconnects cannot be considered as ideal lumped elements (resistors, capacitors and inductors) but must be representative as distributed line elements having transmission line characteristic impedances and propagation delays. In addition, the transmission line characteristics are typically not uniform along the interconnect and therefore must be represented using several transmission line segments having different characteristic impedance and propagation delays.
Of course, the advantages of using TDR may become moot and not be realized if proper connections and circuit setup of the TDR equipment cannot be achieved. Because of small geometries and complexity of some printed circuit boards or IC packages, small probes for contacting selected points or parts on the circuits are used to introduce test signals to the interconnect trace or conductor. Further, it is typically necessary to reference the test signal to a ground plane or suitable reference conductor. Of course, adding ground planes to IC packages and/or PC boards may not even be possible, much less cost effective. Further, choosing a suitable and repeatable reference conductor may also present difficulties for complicated packages or PC boards.
Therefore, it would be advantageous to provide a test probe arrangement where a uniform ground plane is always present and which allows the user to observe the probe as it contacts the circuit under test.
The present invention discloses a test probe for use with TDR equipment which provides its own uniform ground plane and allows the contact point on the probe to be observed as it is being used. More specifically, a probe for use with a TDR system is disclosed and comprises a support portion or member from a non-electrically conductive material. The support member is typically elongated so as to define an axis between a first end and a contact end. A conductive sheet member such as for example a wire mesh is attached to the support member proximate the contact end. The conductive sheet member is attached to the support member such that it extends away from the axis of the support member. Typically, the conductive sheet member extends substantially radially (that is, the conductive sheet is substantially orthogonal) to the axis. An electrically conductive contact point or portion for contacting a selected location of a conductor or trace under test is supported at the contact end of the support member and is electrically isolated from the conductive sheet. There is also included a first electrical conductor between the conductive contact point or portion and a first electrical input and a second electrical conductor connected between the sheet member and a second electrical input. The first and second electrical conductors may be a coaxial cable. The first electrical input is typically a pulse of a known rise time and direction used in time domain reflectometry, and the conductive sheet such as the wire mesh serves as a built-in uniform reference ground plane. The conductive member, such as a wire mesh, may be selected such that the open areas between the weave or the perforations is sufficient for light to pass through and for a user to observe the end of the contact probe as it is being used. Alternatively, specific apertures may be defined in the conductive sheet member to allow the user to observe the contact point of the probe. The probe is, of course, typically used with a time domain reflectometer system which provides xe2x80x9cincidentxe2x80x9d pulses at the first output with respect to the ground plane. An oscilloscope is typically used to monitor the xe2x80x9cincidentxe2x80x9d pulse and the reflective pulse(s) to analyze the circuit or conductor and/or test for faults.